I think the main reason it works like this is because you need the marketing team to be able to edit the website without having to rely on the engineering team.

This is not a concern for solo/bootstrapped projects, and it's way more convenient to have everything at the same place imo.

The second reason is the marketing website needs to be more optimized for SEO, so if what you're using to build the app has no SSR support it might not be the best choice.